RE: Ornithischia/Saurischia Ilium Mass Distribution Hypothesis

["Mike Milbocker" <mmilbocker@psdllc.com>]

With regards to the mass distribution hypothesis, Andrew A. Farke writes:

>What happens with other ankylosaur or ceratopsid ilia (e.g., Sauropelta,
etc.)? Ilia are very "three-dimensional" elements in these taxa, with a
significant lateral projection. Could you estimate the three-dimensional
moment of inertia based on orthogonal views of the same specimen? Based on
the few illustrations (and recollections of actual specimens) of
ankylosaurs, the center of mass of the ilium could be anteriorly placed.

At first I was very concerned about this group, precisely because their ilia
are 3-D. Many in the group you cite have a significant lateral deflection
posteriorly (not anteriorly). For example, see the ankylosaurus at
www.mathematical.com/ dinoankylosaur.html. It's true, in side view the
anterior portion looks larger. The best approach would be to make a cast of
the ilium and follow the procedure as described. At least for the
ankylosaurus, I was able to convince myself from side and dorsal views that
it possesses an unambiguous ornithischian ilium.

>What's the significance of the differences between Saurischia and
Ornithischia? Is it related to posture? To muscle origin and insertion? Is
it somehow related to the position of the sacro-iliac joint? Is there no
significance at all?

I suspect you are not asking me about the importance of the division of the
dinosauria into two primary groups, rather I suspect you are asking, "What
is the importance of the mass distribution in the ilium, functionally, in
defining the locomotive and possibly behavioral distinctions between the two
groups." To begin to answer this question I think it would be rewarding to
quantify the anterior/posterior ilium mass ratio for a large number of
examples in both categories. Speaking generally, and I may be wrong here,
theropods yield the highest ratio >>1, sauropods approx. 1, Ornithischian <<
1, through there are borderline cases in the Ornithischia as well. Now
speculating, this ratio appears to be related to max. stresses transferred
from the thoracic region to the ilium via the synsacrum (hence the minor
importance functionally of the pubis and ischium and their orientations).
Incidentally, the position of the pubis and ischium, I would suggest, has
more to do with tail function, e.g., whether the tail is used as a stiff
navigational member or a flexible, possibly defensive member. Does the ilium
mass distribution support a rearing Sauropod, I wonder?

>If you have a sample of ilia from the same taxon (or a group of related
taxa, such as hadrosaurines), how much intragroup variation is there?
What's the range of variation, in general?

I do not know, but this variability can be quantified and statistical
measures applied which may prove predictive.

And, what happens if you use photographs versus drawings?

Photographs are preferred, and from at least 2 orthogonal directions. An MRI
would be better. The MRI might allow one to take into consideration mass
density variations, which I suspect would remove some borderline cases since
bone density is known to be increased by stress.

How about extant birds?

For the approximately 20 I checked, the hypothesis yields unambigous
Saurischian results. Birds introduce a complication since the ilium,
ischium, and pubis are fused. Thus separating them introduces subjectivity.


Thanks for the thoughtful comments, regards

Mike Milbocker